Unacceptable leg length inequalities, mis-sized lateral offsets, and joint dislocations are possible adverse outcomes of total hip replacement surgeries. Leg length inequalities and mis-sized lateral offsets (including, particularly, insufficient lateral offsets) can result in a persistent limp for patients who have undergone total hip replacement surgeries. Also, joint dislocations are more likely to occur if the soft tissue tension across the replaced joint is too lax as a result of a shortened limb and/or an insufficient lateral offset.
It is, therefore, generally desirable to maintain the leg length, and the amount of lateral offset, which existed prior to the total hip replacement surgery. Sometimes, however, it may be desirable during total hip replacement surgery to change the leg length, and/or the amount of lateral offset, so as to compensate for deficiencies existing in the hip joint prior to the total hip replacement surgery.
The desired leg length, and the desired lateral offset, are achieved during the total hip replacement surgery by selecting a femoral head prosthesis which has the appropriate neck length and geometry, and/or by varying the amount of bone resection performed on the femur.
To achieve the desired leg length and lateral offset during total hip replacement surgery, the surgeon typically determines the initial (i.e., pre-dislocation) position of the femur relative to the ilium, based on reference points selected on each of these bones. A number of different devices and methods for measuring the position of the femur relative to the ilium are known in the art. These existing devices and methods are generally used in conjunction with one or more reference pins which are inserted into a convenient location in the ilium, e.g., typically a few centimeters above the acetabulum. The position of the patient's femur is then measured off the aforementioned one or more reference pins, by measuring to an appropriate reference point on the femur, e.g., typically in the area of the greater trochanter. It should be appreciated that the femoral reference point is typically located in the area of the greater trochanter inasmuch as the greater trochanter is not removed during femoral resection, yet is on the upper part of the femur and hence reasonably close to the patient's ilium so as to facilitate measurement.
By way of example, various devices and methods for measuring the position of the femur relative to the ilium are disclosed in U.S. Pat. Nos. 5,122,145; 5,423,828; 5,435,321; 5,603,717; 5,616,147; 5,700,268; and 5,755,794.
Unfortunately, however, all of the existing devices and methods for measuring the position of the femur relative to the ilium tend to suffer from one or more significant disadvantages, on the one hand, some of the existing devices only measure the longitudinal displacement of the femur, and they do not measure the lateral offset of the femur. On the other hand, in those devices which can measure both the longitudinal displacement and lateral offset of the femur, the devices tend to be relatively bulky and complicated to install and use.